The present invention relates to a system which processes an image such as a document as binary image data. More specifically, it relates to an image processing system having a function to input or store an image, a function to perform magnification, reduction, rotation or other operations with respect to an image, and a function to display an image on a display or print out an image from a printer, wherein a line image such as character, a dither image (hereinafter referred to a pseudo half tone image), or a composite image composed of line image and pseudo half tone image is subjected to optimum image processing to allow an image of high quality to be outputted.
Elements constituting a document includes a line image such as character, a gray scale image (hereinafter referred to a half tone image) such as photograph, and a screened half tone image generally used in a printed matter. In processing these images as binary image data constructed of black and white pixels, commonly adopted in the art is a system for binarizing a line image region by using a fixed threshold, and binarizing a photograph or a screened half tone image by using a dithering process (hereinafter referred to a pseudo half tone process). Thus, an obtained binary image characteristic differs greatly depending upon the applied binarization process.
However, a conventional apparatus for processing a document image is directed only to a line image, and a function to improve the quality of a pseudo half tone image has not been considered at all.
Further, although a conventional apparatus is provided as an image processing function with an affine transformation function such as magnification/reduction. However, such affine transformation function has been directed to a line image.
The principle of conventional affine transformation is as follows: Assuming that an original image is reduced in size to 2/3, first, pixels Q(x, y) of a reproducing image are disposed at 3/2 intervals of original image pixels P(x, y) as shown in FIG. 2. The values of pixels of a reproducing image such as at Q(0, 0) and Q(2, 0) are determined in accordance with the values of pixels of an original image at P(0, 0) and P(3, 0), whereas the values of pixels positioned between original image pixels such as at Q(1, 1) are determined through an interpolation process. A conventional apparatus directed to a line image has generally adopted such as an interpolation process a logical OR method, and a nearest point method. For example, refer to a paper "Estimation of Performance of Various Binary Image Magnification/Reduction Processes and Method of Improving Process Speed", by Hiroshi Masajima, the Proceedings of Information Processing Society in Japan. Vol. 126, No. 5, September 1985, pp. 920 to 925.
A magnification/reduction process for a pseudo half tone image on the other hand is known disclosed in "Study on An Enlargement and Reduction of Dithered Images", by Kim et al, the lecture paper 1-194 (November 1987) at the national meeting of the information and system section of the Institute of Electronics, Information and Communication Engineers of Japan, 1987. In this paper, a multivalued image is estimated from a pseudo half tone image binarized through an ordered dither method, the multivalued image is subjected to magnification or reduction and, thereafter, re-binarized through the ordered dither method. A known example of estimating a multivalued image from a pseudo half tone image is disclosed in JP-A-62-117072. According to this estimation, a ratio of black pixels relative to white pixels within a variable size window is used as an estimated value. The window size used in such estimation is selected based on a similarity of an image re-binarized from the estimated multivalued image to the original image.
In the present specification, an image composed of line image and pseudo half tone image is called a "composite image".
As described above, for processing a document as a binary image, there are two types of images, namely, a line image and a pseudo half tone image, which have a different image characteristic.
As a pseudo half tone process, an ordered dither method which uses a periodically changing threshold value is commonly used. However, this method when applied to a screened half tone image results in a poor image quality because moire is generated. In view of this, as a pseudo half tone process for a screened half tone image, a method called an average error dispersion method for example has been used.
Consequently, it becomes essential to provide an image processing system applicable to all types of images of a document obtained through various binarization processes, in order to allow various image processings. In addition, it is necessary to adjust the characteristic inherent in each of various images having a different characteristic, in accordance with the characteristic of an output device, in order to display/output an image of high quality.
Nevertheless, most of conventional systems for processing binary images have as their main object to process an ordinary line image such as character and figure. Thus, a function to improve the quality of a pseudo half tone image has not been considered at all, thus resulting in the following problems associated with image display/output.
Generally, a binary image output device such as display or printer is not linear in its density characteristic of an outputted pseudo half tone image. A conventional means for compensating for a nonlinear density characteristic, however, is directed to a multivalued image, and cannot be applied to an image after subjected to a binarization process.
Apart from the above, there occurs a case where an optimum pseudo half tone process must be changed depending upon an output device to which a pseudo half tone image is outputted. For instance, a certain laser beam printer (LBP) cannot display correctly a single isolated black or white pixel. If a Bayer type dither image having a number of isolated black and white pixels is outputted to such an apparatus, the image quality is considerably degraded. An effective countermeasure against this problem, however, is not present up to date.
Further, the number of pixels of an image display device such as CRT display is generally smaller than that of an image input device such as image scanner. In consideration of this, the number of pixels of an image to be outputted is reduced if the entirety of an inputted image is to be displayed. With a conventional apparatus, image data to be displayed are simply thinned in such a case, while also seeking to improve the display speed. However, thinning a pseudo half tone image results in considerably lowering the image quality. As a result, a pseudo half tone image having a high resolution is displayed sometimes as having a lower image quality than that of an image having the same resolution as that of a display device. If all image data are inputted, the display speed will be lowered. Thus, the image quality and the display speed are contradictory to each other. Although an effective countermeasure against this problem has been desired, there is no such countermeasure up to date.
Furthermore, the following problems also arise during image processing such as magnification/reduction.
First, if conventional image processing for a line image is applied to a pseudo half tone image, blank or crushed black pixels appear during an interpolation process thus to greatly degrade the image quality.
On the other hand, the conventional image processing for a pseudo half tone image incorporated in the foregoing as a known prior art performs a pseudo half tone process during the image processing. Therefore, if this process is applied to a composite image, the quality of the line image region thereof is deteriorated. Meanwhile, a coding process is generally effected for recording/communications of binary data. However, a coding process commonly used in the art has a low data compression efficiency for a pseudo half tone image. Therefore, if the conventional image processing for a pseudo half tone image is applied to a line image, the amount of data after coding increases greatly.
Still further, the conventional image processing for a pseudo half tone image has been directed to only an ordered dither image among pseudo half tone images. Each pseudo half tone image has a different characteristic depending upon the applied binarization process. Therefore, if the method, for example, of estimating multivalued data from the number of black pixels within a variable size window, incorporated in the foregoing as a known prior art, is applied to a pseudo half tone image other than an ordered dither image, it is not possible to correctly determine the window size and obtain an effective, estimated value.